1. Field of the Invention
The present invention relates in general to aids used in the field of forming ceramics, ceramic bodies or pieces produced using such forming aids, and a process of forming ceramic articles or products with such aids. More particularly, the invention is concerned with a forming aid which contains mineral oil as a major component, and which is suitably used for forming a ceramic material into a desired shape. The invention is further concerned with formed ceramic bodies obtained by using such a forming aid, and a process of producing ceramic products (fired ceramic bodies) from such formed ceramic bodies.
2. Discussion of the Related Art and Problems Solved by the Invention
Silicon nitride, silicon carbide and other silicon ceramics, alumina and zirconia ceramics and similar ceramics are more stable at an elevated temperature (more resistant to heat and thermal stresses), and more resistant to oxidation corrosion and creep deformation, than metals. Active efforts have been made to utilize these excellent properties of such ceramic materials for engine parts such as turbocharger rotors.
In the art of fabricating such ceramic parts, there has been practiced a process which comprises: adding to a suitably prepared ceramic material an organic binder (forming aid) whose major component is a resin or wax, and kneading the ceramic material and the organic binder into a mixture; forming the mixture into a formed body of a desired shape; and heating the formed body so as to remove the binder and fire the formed body into a fired ceramic body, i.e., a desired ceramic product. An injection-molding process is suitably used for efficient, highly accurate production of parts having a complicated configuration, in particular, the rotors referred to above.
As the forming aids, the following organic binders are commonly used in the process indicated above: an organic binder which includes as a major component or components a thermoplastic resin such as polystyrene, polyethylene, co-polymer of ethylene and vinyl acetate, and atactic polypropylene; an organic binder whose major component or components consist of a wax such as paraffin wax and microcrystalline wax; an organic binder including a sublimable material as a major component; and an organic binder which includes two or more materials selected from among the preceding materials. For example, the use of atactic polypropylene and polyethylene as major components of an organic binder is proposed in Laid-Open Publication 49-114610 of Japanese Patent Application, and the use of an organic binder including a wax as a major component is proposed in Laid-Open Publication 58-223662 of Japanese Patent Application. Further, Laid-Open Publication 57-156365 of Japanese Patent Application discloses an organic binder which includes atactic polypropylene and a sublimable material as major components. While these various organic binders have been used in the art, the following requirements must be satisfied to provide flawless ceramic products:
a) high level of formability of a mixture of a ceramic material and an organic binder, to assure exact formation of the ceramic material to an intended shape without geometrical defects;
b) freedom from damage or defects of the formed ceramic piece or body during removal of the organic binder; and
c) sufficiently high homogeneity and density of the fired structure or product.
However, the conventionally used organic binders whose major components are selected from thermoplastic resins, waxes and sublimable materials do not necessarily satisfy all of the requirements indicated above. For example, an organic binder which is excellent in formability suffers from a drawback that it tends to cause cracking of the formed ceramic body during removal of the binder. If a thermoplastic resin is used in a relatively large amount, the formed ceramic body may be broken or damaged due to a pressure of gases which are produced in a large volume due to decomposition of the resin, that is, the resin tends to cause cracking of the ceramic body during removal of the binder. Therefore, a complete removal of the binder is difficult. In the case where a large amount of wax is used as a binder, the rate of removal of the wax is very low since the removal is effected by a viscous flow of the wax. In this case, the formed body tends to have a fairly high residual strain or permanent set, which may cause the formed body to crack during removal of the wax, and/or cause the fired body to have defects like marks or traces of flow of the wax. Stated in more detail, wax-based binders contained in a formed ceramic body will initiate solidication or curing accompanying a large degree of volumetric reduction, if and immediately after the formed ceramic body in an injection mold or other mold is rapidly cooled. For this reason, the ceramic body containing a binder wax is easily subject to a permanent set or residual strain, which may result in the formed body being cracked upon removal of the wax, or the fired ceramic body having the flow-mark like defects.
It is also recognized that any conventionally used binders mixed in a formed ceramic body must be removed from the ceramic body as slowly as possible, in order to protect the ceramic body from cracking during the removal of the binders. For instance, a formed ceramic body containing a conventional binder is heated at a rate as low as 1.degree.-3.degree.C./hr., to remove the binder from the ceramic body without cracking. Thus, the known process of fabricating ceramic products using the conventional binder suffers from a considerably low level of productivity.
In the light of the foregoing inconveniences experienced in the art, there has been a need to develop a binder (forming aid) which permits a ceramic body to be formed and fired into a desired ceramic article, without cracking or defects, and which can be easily and quickly removed from the ceramic body.